1. Field of the Invention
The present invention relates generally to the field of charging control circuitry that acts to effect charging of a storage battery in response to the actual battery voltage and battery temperature. The present invention also relates to voltage monitoring circuits and more particularly to circuits that monitor voltage as a function of temperature.
2. Discussion of the Prior Art
Storage batteries, such as those used in automobile electrical systems, must be recharged after use to restore the full electrical capacity of the battery. While it is desirable to charge the storage battery fully, it is possible to damage the battery by over-charging it. An automobile battery charging system therefore must monitor the battery voltage and charge the battery when the voltage is low, but limit charging as the battery voltage approaches a predetermined limit. This predetermined limit is defined by the gassing voltage, that is, the voltage at which the electrolyte is dissociated and evolves hydrogen and oxygen. The state of charge of a storage battery is maximized as the battery voltage approaches the gassing voltage. The gassing voltage depends upon the temperature of the battery and is inversely proportional to battery temperature. A "cold" battery will have a high gassing voltage, while a "warm" battery will have a lower gassing voltage. For example, FIG. 1 shows the gassing voltage versus temperature of a typical 12 volt automobile storage battery. When the battery voltage is higher than the gassing voltage for a particular temperature, the battery electrolyte will begin to dissociate. Since the state of charge of the battery is maximized just below the gassing voltage, as shown in FIG. 1, a "cold" battery charged only to the "warm" battery gassing voltage would be severely below capacity. Therefore, in order to maintain the battery in the highest state of charge, the battery voltage should track the gassing voltage curve.
One prior art charging system has a temperature-sensitive characteristic which limits the battery charging voltage as a function of the temperature of the voltage regulator. However, the voltage regulator is often physically separated from the battery and is therefore frequently at a different temperature than the battery. For example, if the voltage regulator is mounted on or in the alternator, it senses the alternator temperature which is generally higher than the storage battery temperature. In addition, when the alternator is heavily loaded by various electrical accessories within an automobile, the alternator may experience a significant temperature rise. The elevated temperature will cause the voltage regulator to limit the battery charging voltage and thereby cause the battery to be undercharged. This effect may be exaggerated in cold weather where the demands on the battery for starting the engine and operation of vehicle electrical accessories are very high. The high load on the voltage regulator and alternator may increase the temperature of the voltage regulator and thus exaggerate the temperature differential between the "cold" battery and the "hot" voltage regulator. The result is that the battery is severely undercharged during cold weather, which is just the time when the battery should be in its highest state of charge due to the high electrical loads on the vehicle electrical system.
Another prior art method for measuring battery temperature is through the use of a temperature-sensitive resistor placed in close proximity to the battery. However, this resistor would be measured by the alternator or voltage regulator and therefore requires additional special wiring in order to insure an accurate measurement.
It is therefore an object of the present invention to provide a temperature-sensitive voltage monitoring apparatus which may be used to determine when a storage battery should be charged at the measured combination of battery voltage and battery temperature.
Another object of the present invention is to provide a charging system control apparatus that maintains a storage battery in its highest state of charge for a given battery temperature.
Yet another object of the present invention is to provide a charging control system apparatus that limits charging system voltage as a function of the voltage regulator or the alternator temperature.
Still another object of the present invention is to provide a charging system control apparatus that regulates the charging system voltage as a function of electrical accessory use.
Another object of the present invention is to provide a temperature sensitive voltage monitoring apparatus that may be fabricated using integrated circuit technology.
A still further object of the present invention is to provide an alternator control apparatus that may be fabricated using integrated circuit technology.